Recyclable copper catalysts based on imidazolium-tagged C₂-symmetric bis(oxazoline) and their application in D–A reactions in ionic liquids

Functional imidazolium ionic liquids have been developed as a new class of versatile catalysts. C₂-symmetric and unsymmetric imidazolium-tagged bis(oxazoline) ligands were prepared, and the anions of the ligands were altered by ion-exchange reactions. The catalysts based on the new ligands and Cu(OTf)₂ were applied in asymmetric Diels–Alder reactions between N-acryloyl/N-crotonoyloxazolidinones 15 and 1,3-cyclohexandiene/cyclopentadiene 16 in different ionic liquids and in dichloromethane (DCM). The catalysts achieved a high level of activity and enantioselectivity, as well as good recyclability: cycloadduct (S)-17ab was attained at 98% conversion and 97% ee in [Bmim]NTf₂. Moreover, the catalyst could be recycled 20 times without an obvious loss of activity or enantioselectivity. By comparison, we deduced that the C₂ symmetry of the new ligands was crucial for obtaining high ee values. Toxicity studies of the ligands were performed for the first time.